Custom captive portal

I'm trying to set up a WLAN with a captive portal that points off to an internal website on which users can change their AD passwords. We have around 700 students, most of whom have their own devices and need to be able to manage AD credentials off these devices.

The portal itself works fine, but how can I set up a WLAN that redirects to this internal portal only so that they can change their passwords? I'm already using guest services to redirect them to Zero IT activation on another WLAN which works brilliantly.

The HotSpot WLAN will direct users to an external auth server, where you can
customize the landing page, perform AD pw change/authentication, and drop
them on your desired page, or their original homepage URL.